Electrochemical behaviour of (protoporphyrinato IX)iron(III) encapsulated in aqueous surfactant micelles

Abstract

The electrochemical behaviour of iron protoporphyrinate IX (hemin) [(3,7,12,17-tetramethyl-8,13divinylporphyrin-2,18-dipropanoato)iron()] in aqueous sodium dodecyl sulfate (sds), hexadecyltrimethylammonium bromide or Triton X-100 surfactant micellar solution was investigated by cyclic voltammetry (CV) and Osteryoung square-wave voltammetry (OSWV) techniques. The dependence of the midpoint potential on the concentration and on the nature of the surfactant shows that the potentials shift anodically with respect to water or aqueous ethanol. The midpoint potentials of hemin at pH 7.0 vs. normal hydrogen electrode are Ϫ52 mV in NMe 3 (C 16 H 33 )Br, Ϫ112 mV in Triton X-100, Ϫ152 mV in sds and Ϫ190 mV in ethanol-water (1 : 1, v/v) solutions. Thus the hydrophobic effect of the micelle gives a positive shift of the midpoint potential. The maximum positive shift in surfactants (with respect to water) of ca. ϩ120 mV was found in the micelles. The trend in the anodic shift is EtOH-water < sds < Triton X-100 < NMe 3 (C 16 H 33 )Br. The diffusion coefficients of the hemin complex in the micelles are an order of magnitude smaller than that of monomeric hemin in aqueous ethanolic media. The rates of heterogeneous electron transfer at the glassy carbon electrode were found to be smaller in the micelles as compared to those in aqueous ethanolic media. The midpoint potential of hemin monomers encapsulated in aqueous surfactant micelles shows a pH dependence with ∆E/∆pH ca. Ϫ59 mV indicating that electron transfer at the iron site is influenced by the uptake of protons at the axial ligand.

Electrosynthesis of SER-Active Silver Nanopillar Electrode Arrays

“…The average formal redox potential from several preparations is about -350 ( 10 mV, which is very close to the value for hemin in solution. 37 Upon increasing the scan rate from 20 to 1000 mV s -1 peak currents increase linearly, as expected for surface confined species (Figure 5B). From the separation between the anodic and cathodic peaks as a function of the scan rate (Figure 5C), and using Laviron's formalism, 38 we estimate a standard electron transfer rate constant k°) 16 s -1 for the immobilized hemin.…”

Heme Release in Myoglobin−DDAB Films and Its Role in Electrochemical NO Reduction

“…Various other studies also showed that interactions between surfactants and myoglobin can induce the release of heme groups, − resulting in the incorporation of the heme groups in micelles . In addition, it is known that heme groups incorporated in micelles can exhibit a strong voltammetric response. − Furthermore, similar redox potentials for the Fe III /Fe II couple have been reported for different heme proteins incorporated in DDAB films, and often these potentials do not correspond to the redox potentials of the proteins in solution . In addition, the charge under the voltammetric peaks corresponds to more than one protein monolayer, which combined with the fact that the peaks are reversible up to scan rates of more than 100 mV/s 23,28 would suggest an almost improbably fast protein diffusion within the DDAB layer.…”

Electrochemically and Catalytically Active Reconstituted Horseradish Peroxidase with Ferrocene-Modified Hemin and an Artificial Binding Site

“…It was reported that a reversible electrochemical behavior is typical of the latter. [33,34] Attempts to reproduce such voltammograms both by testing hemin chloride and bubbling nitrogen or argon through the aqueous solution failed. In fact, a decrease in the catalytic current was observed, but cyclic voltammograms unaffected by catalysis have not been obtained.…”